The present invention relates to a method and apparatus for bringing a material having a melting point to an operative melted condition where it can be processed.
It is often desirable to bring a material, such as the caprolactum or diphenylmethane diisocyanate used in a reaction injection molding process, from a solid state at room temperature to an operative melted condition suitable for molding. The material is placed in a tank and heated to the melting point. The material should be agitated as soon as melting begins in order to establish an even temperature throughout the material, to avoid overheating at isolated regions, to avoid degradation of material in contact with the tank walls, and to speed up the melting process.
A tank agitator ordinarily driven by an electric motor is disposed in the tank for agitating the moldable material. If the agitator is activated too long after melting occurs, the moldable material may overheat or degrade. Also, the melting process may not take place as quickly in the absence of agitation. However, if the agitator is prematurely activated before sufficient melting has occurred, the tank agitator may encounter excessive resistance to agitation, thereby overloading or even stalling and damaging the agitator drive, especially if the drive is an electric motor.
Electrical control circuits for sensing overloads are known in the prior art. For example, U.S. Pat. Nos. 3,405,318 to Gilker, 3,416,035 to Lipnitz, and 3,440,491 to Tenenbaum et al. all disclose circuit breakers or interrupters where the breaker is tripped in response to an overload and is then reclosed after a time delay. The circuit breaker may be tripped and reclosed a certain number of times before it is locked out in an open condition. However, none of these references suggest how a moldable material may be brought to an operative melted condition.